Propulsion - Many Roads Go Up

PROPULSION - MANY ROADS GO UP

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HE TECHNOLOGICAL revolution of the past wings, others derive their lift from their body geom- decade and a half has left its imprint on almost etry, others are symmetrical and will produce little or Tevery facet of air and space vehicle development. no lift. Regardless of their mission the performance Nowhere is this more dramatically illustrated than in of each of these vehicles will depend in high degree the vital field of propulsion—key to range, payload, on the effectiveness of its propulsion system. and speed. Many improvements in existing engine schemes can Technical progress since World War II has opened be expected, some of them of a radical nature. It is up innumerable types of possible engine systems com- also possible that, in the decade ahead, some funda- pared to the few that were feasible in the early 1940s. mentally new means of propulsion will appear. At that time reciprocating engines were the prin- Development costs are extremely high. The power cipal type in development. Turbojets, ramjets, and plant normally is the principal expenditure item for rocket engines were just emerging from the labora- any weapon system, and its development can run to tories of a few persistent pioneers. several hundred million dollars. If a major basic- or Today the Air Force is investigating or develop- applied-research program is involved, the total cost ing turbojets, ramjets, liquid-fuel rockets, solid-fuel of obtaining a satisfactory engine can go to a billion rockets, nuclear rockets, nuclear ramjets, nuclear tur- dollars or more. bojets, ion rockets, and plasma rockets. Specific engines Management skill is a key factor, perhaps the most within these general classes include: turborockets, important requirement for orderly and efficient pro- turboramjets, external-burning ramjets, turbofans, lift pulsion programs of the future. The lead times for fans for VTOL use, hybrid rockets using liquid new engines are so long, the needs for men, facilities, and solid propellants, spherical-shaped solid rockets, and dollars are so large, and the technical choices so rockets with plug nozzles, clustered rockets, seg- diverse that there is small room for major error or mented rockets, direct-cycle nuclear turbojets and omission in the space age. The importance of basic ramjets, and indirect-cycle nuclear turbojets and ram- decisions and the long lead times required to repair jets—among others. errors of judgment have been illustrated dramatically The uses to which these engines can be put are as by the construction of "small" engines for US ICBMs. varied as the engines themselves. Many types of recon- These engines perform the ICBM task efficiently but naissance and bombardment vehicles, for use both in do not meet the corollary requirement for large space space and in the atmosphere, are being studied by the boosters. As a result, the US space effort faces a Air Force. Some of these vehicles have recognizable severe payload handicap at least until the mid-1960s.

48 AIR FORCE Magazine • February 1961 Technological progress in the field of aerospace propulsion presents stirring new vistas and weighty new problems. Upcoming decisions in this vital area will have a considerable effect on the nation's military strength, space capabilities, and budgets in the years ahead ...

J. S. Butz, Jr. TECHNICAL EDITOR, AIR FORCE MAGAZINE

PART I — THE AIR-BREATHERS

The extreme payload sensitivity of its booster sys- cult. It will be impossible to develop and operate all tem is the primary reason for the stretchout in design of the technically promising propulsion systems and and development of Project Mercury. The US is vehicles. Many feasible systems must be bypassed. The limited to a total capsule weight of about one ton only certainty is that this sorting-out process and the for its first man-in-space program; the Soviet Union search for the best means of using available resources apparently has a five-ton leeway for the same mis- is certain to grow more difficult with time. sion. Undoubtedly, keeping weight down will become The multitude of possible propulsion systems may a problem with Dyna-Soar. All new types of vehicles be broken down into five general classes to allow a seem to gain weight beyond their original estimates. closer look at some of the technical decisions which The magnitude of the management task for all will have to be made in the near future and during vehicle systems is certain to bring changes in devel- the 1960s. opment organization and procedures. This has been These decisions will have a considerable effect on the theme of suggestions in every critique of the US the military posture of this country, its ability to ex- research and development effort, from Von Karman's plore and control space in the 1970s, and on the na- Toward New Horizons of 1944 to the Stever report of tional budgets. 1959. The five classes of engines are: air-breathers, rockets, The three main suggestions are: ( 1) longer term electric systems, propellant-collection systems, and planning and financing to cover several years or at nuclear and advanced systems. least the complete life of a development project; ( 2 ) The ultimate potential for air-breathing engines does increased specialization and technical competence for not yet appear to be in sight in any speed range and officers in R&D posts which require major changes in for any type of aircraft. Improved designs for light- current personnel policies; (3 ) greater authority and weight VTOL lifting engines and Mach 3 turbofans freedom of action for contractors and military per- are now under development by the military and US sonnel on the working level of research and develop- engine manufacturers. Air-breathing engines which ment programs. Under this arrangement highly placed will operate up to escape speeds of 25,000 mph at commanders and civil authorities will retain close con- altitudes of 250,000 feet or better are now considered trol only over general policy and the selection of major possible by competent engine designers, although programs. there is little experimental proof to back up their Regardless of organizational improvements the main theoretical predictions. Several types of air-breathing task of top management will remain extremely diffi- (Continued on following page)

AIR FORCE Magazine • February 1961 49

PROPULSION CONTINUED COMBUSTION CHAMBER NOZZLE

INLET SHOCK WAVES engines are considered possible at these extreme speeds and altitudes, including turborockets, super- AM sonic combustion ramjets, and external-burning ramjets (see accompanying cuts). However, the course of air-breathing engine devel- ■IIIIIMIMIMIIIIIIIIIIIIIImmimmNm..... opment above Mach 6 or so already has been greatly influenced by management decisions. The National Aeronautics and Space Administration essentially

CONVENTIIONAL SUBSONIC COMBUSTION RAMJET eliminated air-breathing engine research from its program shortly after it was formed around the nucleus of the personnel and facilities of the National Ad- COMBUSTION CHAMBER visory Committee for Aeronautics. The efforts of the - NOZZLE \INLET SHOCK WAVES . group at Lewis Research Center, Cleveland, Ohio, one of the world's largest and most experienced air-breath- V ing engine research teams, was redirected at that time. This was a controversial decision, both within NASA and among power-plant experts throughout the entire western world. The reasoning behind the decision was

111111111■ that no further basic or applied research was needed 111...... miiiiiiiiiiiiiiiiiiim...... by manufacturers to build a new generation of improved turbojets for speeds below Mach 4. This essen-

SUPERSONIC COMBUSTION RAMJET tially has proved to be true because several manufacturers are working in this area, although much of their effort has been of a research rather than a develop- mental nature, and NASA data could have been helpful. INLET SHOCK WAVES ...,y, MOVABLE COWL In the hypersonic engine area, it was decided that I experimental research should be dropped and all effort concentrated on a wide variety of rocket engines, from more or less conventional chemical engines to nuclear and electric rockets. It was believed that the primary requirement in the 1960s would be for rockets. Now it appears that the military will not follow '14411111111 PLUG NOZZLE K ' this policy lead and that perhaps NASA will reverse itself and return to experimental research with hyper- • HYPERSONIC COWL-TYPE RAMJET WITH SUPERSONIC COMBUSTION sonic air-breathing engines. The Air Force is planning to begin applied research leading to a large, one- stage, winged Aerospace Plane that can fly into orbit using air-breathing engines rather than rockets. NASA

COMPRESSOR WHEEL /- TURBINE WHEEL apparently is going to reenter this propulsion field even PITOT INLET / SHAFT NOZZLE \ , if most of the research work has to be done under con- - --- III --.- '' ,-1-----' ;I/WM: tract,with Lewis Research Center furnishing the management personnel. I i

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FUEL INJECTOR- ROCKET CHAMBER ' Schematic drawings of five types of air-breathing engines COMBUSTION CHAMBER which have potential usefulness at hypersonic speeds are shown at left. The air flow inside the "conventional" ramjet at the top is slowed down to subsonic speeds. This type of engine probably will not be feasible above Mach TURBOROCKET ENGINE 6 or 7 because its internal temperatures will be so high that cooling will be impractical with known methods. The air flow in the ramjet second from the top is supersonic in all portions of the engine except the nozzle throat. WING OR UNDER SURFACE This results in a cooler-running engine than the "conven- OF FUSELAGE tional" ramjet. Further research will be required to prove \ \ that stable combustion can be maintained in a supersonic \ \ rDIRECTION ' - FUEL INJECTION airstream. The middle drawing shows a type of ramjet that POINT \ OF FLOW \ will operate up to speeds of 18,000 mph, according to the ---1.-. -• ■• predictions of some US experts. The cowl around the engine would be movable so that the optimum inlet flow 4 conditions could be maintained. The engine would be / FORWARD SHOCK WAVE cooled primarily by radiation from the surfaces behind the /1 --HEAT ADDITION ZONE cowl. Another radiation-cooled 18,000-mph engine is the FLAME FRONT external-burning ramjet in the lower sketch. Research has shown that external burning is practical at supersonic speeds, but further work will be necessary to prove the technique above Mach 5. The turborocket, second from the EXTERNAL-BURNING HYPERSONIC RAMJET bottom, is one of several types of hybrid, air-breathers which have been experimented with on a small scale.

50 AIR FORCE Magazine • February 1961 The hypersonic vehicle at the left was suggested by Antonio Ferri last year. It would be able to accelerate to 18,000 mph and even more and fly into orbit. The entire undersurface of the vehicle is its engine. Both the external-burning ramjet and the ramjet with small cowl (see opposite page) would be feasible for this installa- tion. Turbojet engines would be used for takeoff and acceleration to Mach 3. This Aerospace Plane would use a high-energy fuel such as hydrogen.

During the two-year hiatus in NASA's experimental on the total cost and ultimate success of any hyper- activities small-scale experimental research in hypersonic orbital airplane. sonic engine technology has been pursued at the Ap- Technically, almost every problem with the hyper- plied Physics Laboratory of Johns Hopkins Univer- sonic engine arises from high temperatures. A number sity, at the National Bureau of Standards, by several of proposals are being investigated to reduce the op- manufacturers—primarily under modest military con- erating temperatures of these engines. One of the most tracts—and by a few small research firms. important is supersonic combustion (referring to the But there is still a great deal of basic propulsion speed of the airstream in the engine, not to the speed design data which must be gathered before the de- of the vehicle). On convention ramjets of the type tailed planning of hypersonic aircraft can proceed on now operating on the Bomarc and other supersonic a firm basis. The management decision to gather this missiles the air entering the engine is slowed to sub- data through large-scale basic- and applied-research sonic speeds in the combustion chamber. Burning fuel programs is going to be a big and costly step. There in a subsonic airstream is much more efficient than are few facilities in the US at which fairly large-scale burning in a supersonic stream. engines can be tested at supersonic speeds and none However, it is possible to reduce the temperature in for hypersonic speeds. The supersonic propulsion wind the engine inlet and combustion chamber by about tunnel at the Arnold Engineering Development Cen- one-half (from more than 4,000 degrees F to about ter, Tullahoma, Tenn., is the largest, and it will ac- 2,000 degrees F in a Mach 8 ramjet) if the airstream commodate engines of several thousand pounds' thrust is not slowed down to subsonic velocities. Slowing the at speeds of Mach 5. This tunnel will be a valuable engine airstream down increases its pressure and tem- development tool, but a hypersonic-research program perature and also raises the rate at which its heat is undoubtedly will require as well the extensive use transferred to the engine wall, so that supersonic com- of large flight-test vehicles. bustion is a very attractive procedure. Estimates of the cost of gathering the necessary The relative inefficiency of supersonic combustion design data for hypersonic engines are matters for de- is overcome at hypersonic speeds by a big improve- bate. Some figures run to several hundred million dol- ment in inlet duct efficiency because the engine air lars for the supporting research alone, with the total need not be slowed and compressed to such a great ex- cost of developing a single type of operational hyper- tent. Temperatures in the supersonic combustion ram- sonic air-breathing engine going to a billion dollars or jet reach those in a comparable subsonic combustion more. These sums are exclusive of the vehicle develop- ramjet at only one point, the nozzle throat. This gen- ment costs and the costs of aerodynamic and structural eral lowering of engine air temperature also reduces research. the tendency of the air to disassociate and soak up Other estimates, which come primarily from outside energy which might be used to produce thrust. The of industry, indicate that the necessary research andl disassociation loss occurs when the temperature of the development programs can be accomplished less ex- air molecules gets high enough to break the molecular pensively—on the premise that the goals be carefully bond and split them into individual atoms. defined and the program allowed to proceed at its At high supersonic speeds the total effect of super- natural pace. Adequate funding would have to be sonic combustion in a properly designed ramjet ap- made available from the start rather than sporadically parently will be a more efficient and cooler-running through a review system by outside agencies. Under engine. At low hypersonic speeds, from about Mach these conditions the estimates are for a total orbital 5 to 10, the supersonic combustion ramjet is not airplane cost of about $1 billion with half of the ex- feasible, but the supersonic combustion engine appears penditure on the power plant. to be a workable device. At moderate and high hyper- Regardless of which estimates are correct, it is cer- sonic speeds ( above Mach 10 or 12 ) the temperatures tain that top-level management in the Administration, the supersonic combustion engine get too high for the military, and industry will have a great influence (Continued on following page)

AIR FORCE Magazine • February 1961 51 PROPULSION CONTINUED known materials and cooling methods, and a complete a special shaping of the engine duct. Another is a change in configuration apparently will be required. turboramjet engine (see page 50) in which the exhaust One type of configuration receiving study is the products of a rocket engine may be used as fuels external-burning ramjet which amounts to turning the in an air combustion process. This double reaction engine inside out so that its hot surfaces may be cooled process can be adjusted to achieve an engine tier- by radiation. An external-burning ramjet of typical formance somewhere close to either the high specific geometry is shown on page 50. The combustion air is impulse of the turbojet or the high thrust and low compressed by the bow shock and slowed down to weight of the rocket, or to meet a compromise set of supersonic speed. The fuel is added at the point of performance requirements somewhere in between. maximum thickness, and shape is adjusted so that all New fuels are an important requirement for hyper- of it is burned in the dotted area behind the rearward- sonic engines. Petroleum hydrocarbon fuels will not facing surface. The static pressure is raised in this be useful because they are thermally unstable at the region by the fuel combustion, and lift as well as temperatures experienced at about Mach 6 and above, thrust is produced on the slanting surface. For this and they vaporize and leave deposits on fuel-injection reason it is more difficult than usual to separate the systems, etc. engine performance from the airframe performance. Hydrocarbon fuels also provide much less range than Through its significant lift production the external- the high-energy fuels such as the boron-hydrides and burning ramjet materially improves the lift/drag ratio hydrogen. The high-energy fuels have higher heats of and the aerodynamic efficiency of a hypersonic air- combustion than hydrocarbons, and they burn effi- craft. ciently at a lower fuel/air ratio to achieve a given Operationally, it has been proposed by Breitieser thrust. Therefore, for a given fuel weight they provide and Morris of NASA to run the external-burning ram- more range. jet "fuel-rich" at the higher flight speeds. There are Many fuels have been investigated by the Air Force three main advantages to this procedure: (1) tempera- and other agencies to meet a wide variety of per- tures of the burned gases are lowered because of the formance requirements. These include such especially excess fuel and there is less disassociation; (2 ) a vary- tailored fuels as those with small bits of metal sus- ing fuel rate can be used to maintain a match between pended in them and others designed to form especially inlet and exit flow conditions at most speeds so that stable combustion products. variable engine geometry will not be required (it is Large facilities were constructed to produce boron- possible that a small movable cowl might be required hydride fuels, but their use was canceled because of to maintain efficient operation—see page 50); ( 3 ) the high costs. Today it seems certain that liquid hydro- excess fuel flow can be used to cool the engine and gen will be the basic hypersonic engine fuel. It is possibly the airframe. in plentiful supply, and it has the highest energy per There is one main disadvantage of running the ex- pound of any fuel although it is not as attractive as ternal burning engine "fuel-rich." Its specific impulse some of the other high-energy fuels from all com- is lowered although it remains considerably higher bustion standpoints. than rocket engine specific impulses. Some researchers One of the implications of using hydrogen fuel is say that the specific impulse will remain high enough that unusually large tank space must be provided be- to ensure a satisfactory acceleration margin to speeds cause of the low density of liquid hydrogen. One proof at least Mach 20 or about orbital speed. Others ap- posed hypersonic aircraft configuration which would parently believe that this type of propulsion could be have the necessary tank volume is shown on page 51. used to rapidly accelerate an aerospace vehicle to This aircraft was suggested last year by Antonio Ferri escape velocities as it flies through the upper levels of of the Polytechnic Institute of Brooklyn. It is designed the atmosphere at altitudes up to fifty miles. to fly into orbit using almost its entire undersurface Not all propulsion scientists agree that hypersonic as an engine after taking off from a normal-sized air- engines will operate as described in the previous para- field with turbojet engines. The turbojets would be graphs—or even that these engines are feasible. There stopped and their inlets and exhaust nozzles closed at is no unanimous agreement that supersonic combus- about Mach 3. tion is possible under all necessary conditions even Any management decision to pursue a costly de- though it apparently has been achieved in the labora- velopment program for hypersonic air-breathing en- tory over narrow ranges of speeds and pressures. gines will have to be made against a background of The first goal of any major research and development continuing improvement in rocket-engine performance. program for hypersonic air-breathing engines will be Liquid- and solid-propellant rockets are entering prob- to gather a great deal of basic evidence that there ably their most promising era, and their performance are no substantive holes in any proposed engine sys- as boosters to "fire" or rapidly accelerate payloads into tem. Once this proof is available it will be possible to space or through the atmosphere is improving each sensibly gather engineering design data. year with no sign of abatement. Several other types of hypersonic air-breathing en- The nature of these chemical rocket improvements gines are possible and have received some attention. along with a discussion of electric rockets, nuclear One is the standing wave or detonation ramjet in engines, and air-liquification engines or propellant- which the fuel energy is released by passing the fuel- collection systems for use in the upper atmosphere air mixture through a stationary shock wave created by will be presented next month.—END

52 AIR FORCE Magazine • February 1961